The present invention relates to a digital composite chromakey system for forming a composite television picture.
The chromakey system is used to insert one image into a predetermined color portion of another image frame. From the projected image shown in FIG. 1(a), more specifically, a key signal corresponding to a predetermined color component, e.g., color (i), is prepared and used to insert the projected image shown in FIG. 1(b) into the color component portion (i) in FIG. 1(a) to form the composite picture shown in FIG. 1(c)
When the composite picture frame is being formed by a chromakey system, the boundary of the two images combined with each other may change gradually from one image to the other one to provide a more natural composite effect. Such a manner of forming the composite image is called "soft chromakey."
A conventional analog chromakey system will now be described with reference to FIG. 2. The color-television signal of the projected image shown in FIG. 1(a) is supplied through terminal A to phase detector 11 and first gate 14. In phase detector 11, the phase of the color signal in the color-television signal is detected using a reference signal from a reference signal generator 10. The output of the phase detector is called the "phase detected signal" and is in a first state while the phase of the color signal corresponds to that of the reference signal.
The phase detected signal is supplied to a key signal generator 12 which generates a key signal while the phase detected signal is in the first state. This key signal corresponds to the predetermined color portion of the color television signal, for example the color blue.
The generated key signal is an input to NOT-circuit 13 and second gate circuit 15. The inverted key signal from NOT-circuit 13 is supplied to first gate 14 which, under the control of the inverted key signal, generates a video signal from which the blue component is removed. The video signal provided from the gate circuit 14 is supplied to a combiner 16.
The key signal from the key signal generator 12 also controls second gate 15 which receives the signal of the projected image shown in FIG. 1(b) from terminal B. As a result, a video signal to be inserted into the blue portion of the color television signal is supplied from gate 15 into combiner 16.
Combiner 16 merges the signals from second gate 15 and first gate 14 to generate the composite signal of the projected image of FIG. 1(c).
This chromakey system involves analog composite processing which unfortunately does not provide good chromakey efficiency because of insufficient color signal separation. Furthermore, in recent years video signals have been digitized to reduce signal deterioration. Without a digital chromakey system, the digitized video signal must be converted into an analog format before chromakey processing and then converted to the digital signal after the chromakey processing. Converting and reconverting requires several circuits and is very complicated.
An object of this invention is a digital chromakey system for image formation which processes a digitized composite signal digitally and thereby reduces the number and complexity of the chromakey circuitry and increases chromakey efficiency.